Process operating at normal pressure for producing oxygen or air enriched with oxygen

ABSTRACT

The invention relates to a process for producing oxygen or air enriched with oxygen, in which process: The air entering the process is cooled in a heat exchanger to near to the condensation point of air and the air is led to a separation unit formed of two or more chambers connected thermally to each other and comprising A and B halves, in which it is divided, using liquidization and evaporation processes, into two or more gaseous fractions. The process air is maintained at normal temperature, except for small pressure differences to maintain the flow of the air and to optimize the process. When the process air flows through the A half of the separation unit, the liquid oxygen concentrate separated from it is evaporated in the B half of the separation unit at such a pressure, lower than normal pressure, that the desired composition is obtained for the oxygen concentrate and the heat of liquidization released in the A half of the unit is transferred to the B half of the unit and is bound in it to the evaporation of the oxygen concentrate. The low-oxygen-content process air leaving the A half of the separation unit is led out of the process through the heat exchanger. The oxygen concentrate evaporated in the B half of the separation unit exits at the lower than normal pressure prevailing in the half in question, either as such, or through a boosting and cooling process to the heat exchanger, and from there out of the process. The liquidization and evaporation processes are carried out in the said separation unit in nearly reversible conditions, in such a way that the gas phase in each process and in each part of the separation unit is nearly in equilibrium with the liquid phase in the same part.

[0001] The present invention relates to a process and an apparatus implementing the process, for producing oxygen or air enriched with oxygen, in which process:

[0002] air at normal pressure is cooled to nearly the condensation point of air (81,5 K) in a heat exchanger,

[0003] the air is led to a separation unit comprising two or more chambers thermally connected to each other, in which it is divided into two or more gaseous fractions using nearly reversible liquidization and evaporation processes, and

[0004] the gas fractions leaving the separation unit are led back to the heat exchanger.

[0005] Cryogenic separation processes are based on fractional distillation and enrichment processes carried out in two columns operating at different pressures, in each of which columns a specific temperature difference is maintained between the bottom of the column and its top condenser. In the classic Linde-Fränkl method, this is implemented by connecting the upper end of the high-pressure column to the lower end of the low-pressure column, the columns operating otherwise adiabatically. The temperature difference between the ‘free’ ends of the columns, i.e. the lower end of the high-pressure column and the upper end of the low-pressure column, indicates how much the process deviates from reversibility. In the Linde-Fränkl process, the difference is 20 K.

[0006] Publications U.S. Pat. No. 5,592,832 and U.S. Pat. No. 5,144,809 disclose the use of non-adiabatic distillation and enrichment columns, i.e. columns that are thermally connected to each other over their entire length, for producing oxygen in the case of the former publication and nitrogen in the case of the latter. By connecting the columns to each other thermally, the temperature difference between the columns is reduced and the energy consumption of the process decreases to some extent.

[0007] Comparatively little energy is saved, because the temperature difference required by the aforementioned distillation is maintained in this case too by exploiting process air boosted to a pressure of several bars and the pressure difference with a ‘low-pressure column’, which is at a lower pressure that is, however, greater than normal pressure, in other words, the work required by the separation is produced by pressurizing all of the process air.

[0008] For reasons of economy, the process pressures of several bars used in the state of the art demand that the separation machine unit be made as small as possible, so that the density of the thermal flux being transferred per unit of volume will be as great as possible. Therefore, the aforesaid publications and their claims are restricted to refer solely to plate heat exchangers, despite the high flow resistance offered by their narrow ducts.

[0009] Publication U.S. Pat. No. 4,192,662 discloses a method for producing liquid oxygen and nitrogen, in which process additional cooling is created using liquid natural gas (LNG).

[0010] The present invention is intended to create an improved process for producing oxygen or air enriched with oxygen. The characteristic features of the process according to the invention are stated in the accompanying Claims.

[0011] The separation unit used in the process of this application operates in principle in the same way as the units used in the state of the art, i.e. it comprises a double column, both halves of which are connected thermally to each other. As in the state of the art, this method too is based on the fact that, when gaseous air and the liquid arising from it during cooling are in a state of equilibrium with each other, the liquid phase becomes enriched with oxygen and the gas phase with nitrogen.

[0012] The process described in this application differs from the state of the art, in that the separation process is moved to a pressure range below normal pressure. The temperature differences required by the distillation process are now created by exploiting the pressure difference between the normal-pressure process air and the low-pressure oxygen fraction produced (as well as the other possible low-pressure fractions), while the separation work is produced by boosting the said low-pressure fractions back to normal pressure. The thermodynamic improvement in the separation process appears in several ways:

[0013] Instead of boosting the process air, only the oxygen concentrate (and its fraction separated in possible enrichment), the volume of which totals about 40% of the volume of the process air, need to be boosted. The low pressure of the process means that a lower pressure ratio than in the state of the art is required to maintain the temperature difference needed for separation, thus further reducing the boosting energy of the oxygen concentrate. In the aforesaid US publication U.S. Pat. No. 5,592,832, the boosting energy of the process air alone is 755 kJ per kilo of oxygen, in an ideal case in which the compressor's efficiency is 85% and all the oxygen in the air can be separated. In the Linde-Fränkl method, the boosting work by itself is about 882 kJ per kilo of oxygen.

[0014] The possibility to use energy-saving separation processes, in which a substantial part of the oxygen is left in the process air. For example: if the air is separated into a 50% oxygen fraction and pure nitrogen, the theoretical separation work at 300 K is 83 kJ per kilo of oxygen. If 6,1% of oxygen is left in the nitrogen fraction, the separation work drops to a value of 58 kJ per kilo of oxygen, i.e. 30% of the energy is saved.

[0015] The possibility of exploiting the high oxygen enrichment coefficients achievable at a low liquidization pressure, so that the energy consuming enrichment of the oxygen fraction, used in the state of the art, will not be required, or will be required to a lesser extent to achieve a specific oxygen content. For example, at a pressure of 5 bar, a liquid phase in equilibrium with gaseous air will be 42% oxygen, in other words, the oxygen is enriched in the liquid phase by a coefficient of 2,0. In a liquid phase in equilibrium with air at a pressure of 1 bar, there will be 52% oxygen, and the enrichment coefficient will be 2,48.

[0016] At a low pressure, it will be possible to operate using a lower temperature difference between the bottom of the ‘high-pressure column’ and the top of the ‘low-pressure column’. When producing both 50% oxygen concentrate and pure oxygen, in the present method this difference is 7,5 K, whereas in the Linde-Fränkl method it is 20 K.

[0017] The invention also provides the following advantages:

[0018] The low pressure level of the process means that the thermal power that has to be inserted in or extracted from a specific volume flow is essentially less than in the state of the art. Therefore solutions can be used, which have wider flow ducts than in the plate heat exchangers referred to in the aforementioned publications, and the power losses due to flow resistance can be reduced. Examples of preferred solutions according to this method are disclosed in the examples of embodiments and the related FIGS. 2-5.

[0019] As the process is carried out at normal pressure and below it, the heavy and expansive pressure vessels required in the state of art will not be needed.

[0020] As, in most of the solutions, the entire separation process is carried out in one double column, the machine unit required will be much simpler than in the state of the art. This can be shown by comparison (U.S. Pat. No. 5,592,832 FIG. 8).

[0021] These solutions are described in the following examples with reference to the accompanying drawings.

[0022]FIG. 1 shows the composition of the gaseous oxygen and nitrogen and the liquid phase in equilibrium with them, as a function of temperature and pressure,

[0023]FIG. 2 shows a diagram of one apparatus according to the invention,

[0024]FIG. 3 shows a diagram of a second apparatus according to the invention,

[0025]FIGS. 4a and 4 b show a third apparatus according to the invention, and

[0026]FIG. 5 shows an apparatus according to the invention using a heat-exchanger circuit.

[0027]FIG. 1 shows the composition of the gaseous mixtures of oxygen and nitrogen and the liquid phase in equilibrium with them, as a function of temperature and pressure. The figure shows that the condensation point of air (21% oxygen) at a pressure of 1 bar is 81,5 K and the liquid in equilibrium with it is 52 mole-% oxygen. The corresponding value at a pressure of 5 bar is 42% oxygen. The enrichment coefficient increases as the absorption pressure decreases.

[0028] In the process equipment according to the invention, shown in FIG. 2, the oxygen is separated from air in a separation unit 10. In this case, the separation unit 10 is divided over its entire height into two parts by a cylindrical wall 13, which permits a heat exchange connection between the first column 11 and the second column 12. Finned plates 38 and 39, which also guide the gas flows, can be used on the thermal-transfer surfaces 13 a and 13 b. The first column 11 acts as the liquidization component, to which air, which is transferred at a pressure of 1 bar by a blower and cooled to 81,5 K by a heat exchanger, is brought to the column 11, from an air-inlet connection 22 in its lower part. The air flows upwards from the lower part of the column 11, cooling and partly liquidizing on the thermal transfer surface 13 a. When the gas phase exits, at 78 K, from the oxygen-enriched air outlet connection 23 fitted to the top of the column 11, only about 6% of oxygen remains in it. The oxygen fraction liquidized on the heat transfer surface 13 a of the column 11 flows down the heat transfer surface 13 a by gravity and is collected in the bottom part of the column 11 and is led onwards through the connection 25. In a fully reversible process, there would be about 52% oxygen in the liquid leaving the bottom of the column 11. In practice, however, a figure of about 50% is achieved. The liquid oxygen fraction is led through a throttle 37, which, together with the head of the liquid, reduces its pressure, onward into the top of the evaporation component of the separation unit 10, i.e. the column 12, from the liquid oxygen fraction feed connection 42. From the top of the column 12, the oxygen-rich liquid flows downwards, to be evaporated back into a gas phase due to the effect of the heat released in the liquidization of the oxygen fraction and transferred to the heat transfer surface 13 b of the first column 11, so that the oxygen fraction interactively cools the nitrogen fraction leaving the column 11. In the column 12, the pressure in the embodiment according to FIG. 1 is 0,4 bar and the temperature at the top of the column 12 is 74 K.

[0029] In addition to the liquid phase evaporated in the column 12, the evaporated gas phase also flows downwards and there is still 50% oxygen, at a temperature of 78,5 K and a pressure of 0,4 bar, in the oxygen fraction that exits in a gaseous form from the bottom of the column 12 through the oxygen outlet connection 24. The oxygen fraction obtained from the column 12 and the nitrogen fraction exhausting at normal pressure from the nitrogen-rich air outlet connection 23 in the upper part of the column 11 are used in the said heat exchanger to cool the air entering the process. From the heat exchanger, the exiting oxygen fraction is boosted to a pressure appropriate to its intended use.

[0030] The temperature difference between the columns 11 and 12 in this embodiment according to the invention is 3-4 K, which is a typical value used in an oxygen plant. As evaporation at a lower temperature binds more heat than is released in the liquidization taking place at a higher temperature, the process in principle produces the cooling power it needs.

[0031] The aforementioned temperature difference represents a deviation from reversibility, in addition to which losses also arise in the separation unit 10 and in the heat exchanger 14. If the process is carried out at normal pressure, the feed of process air to the separation unit 10 requires a blower. The energy it requires is not taken into account here, but part of it is recovered as increased pressure in the separated oxygen fraction. The other losses result in the pressure of the oxygen fraction remaining lower than 0,4 bar, or in changes in the composition of the fractions, so that more nitrogen remains in the oxygen fraction and/or more oxygen remains in the nitrogen fraction. Depending on the size of the separation unit 10 and the details of the process, it may require additional external cooling power. Including the A-side heat exchanger, the pressure loss is at most 0,3 bar, preferably less than 0,2 bar.

[0032] As in other cryogenic methods, the energy consumption of the present process mostly comprises the work of boosting the gases, which can be used to compare the energy consumption of different methods. In the classic Line-Fränkl method, the process air is boosted to a pressure of 5,6 bar, the boosting work being 882 kJ per kilo of oxygen, if the efficiency of the booster is 85%. In an ideal case, the boosting work of the process of U.S. Pat. No. 5,592,832 is 775 kJ per kilo of oxygen. In the table below, these values are compared with the cases depicted in the embodiments 1 and 2 of the present invention. The theoretical pressure conditions are multiplied by 1,5, to cover pressure losses, while the efficiency of the booster is assumed to be 85%. The pressurization of the cold oxygen fraction, shown in FIG. 3, is not used in example 2. Method Boosting work/kg oxygen Linde-Faänkl 882 kJ U.S. Pat. No. 5,592,832 >755 kJ Present invention-example 1 273 kJ (50% concentrate) Present invention-example 2 399 kJ (83% concentrate)

[0033] With these reservations, the basic value of the energy consumption of the process comprises the boosting of the oxygen fraction to a pressure of 1 bar, which, at a temperature of 300 K, requires in theory 115 kJ/Nm3, i.e. 160 kJ/kg O2. The losses of the booster 21 can be covered with water injection. In the booster, the heat transferred to the gas can also be exploited, for example, in the power plant's circulation. If the various losses increase this value by a factor of about 1,5, the energy consumption per kilo of oxygen would be only about 273 kJ/kg O2 (50% concentrate). The corresponding value for an 83% concentrate is 399 kJ/kg O2. The total energy consumption is thus substantially lower than in the aforementioned processes.

[0034] The diagram shown in FIG. 3 shows a second embodiment of the utilization of the process according to the invention, by means of which a greater oxygen content is obtained, compared to the previous embodiment. In this embodiment, the column 11 of the separation unit 10 operates in a manner corresponding to that in the embodiment shown in FIG. 1. Part of the downwards flowing liquid oxygen fraction, brought to the column 12 of the separation unit 10 from the feed connection 42, is evaporated in an enrichment section 27 in the upper part of the column 12. A throttle 21 is used to reduce the pressure of the liquid fraction collected on the bottom of the enrichment section 27, an atomizer 44 then being used to spray the fraction from the upper part of the evaporation section 28 onto the heat-exchanger surface 13 b.

[0035] If there is 50% oxygen in the liquid oxygen fraction, there will then be 83% nitrogen in the gas phase reversibly evaporated from it using the countercurrent principle at 0,4 bar, so that the oxygen is concentrated in the liquid phase. The liquid oxygen fraction leaving the enrichment section is then evaporated to form a final oxygen fraction in the evaporation section 28 of the lower part of the column 12. The gas evaporated from the enrichment section 27 is used in the heat exchanger 14 to cool the air entering the process, and is exhausted using a booster 31. There is 80-95% oxygen in the enriched oxygen fraction, which is also used in the heat exchanger 14 to cool the air entering the process, after which it is exhausted using a booster. In the column 11, the heat released in producing the liquid oxygen fraction transfers from the heat-transfer wall 13 b of the wall of the column 11 to the column 12, the enrichment and evaporation sections 27 and 28 of which are separated from each other.

[0036] The oxygen content of the oxygen fraction produced by means of the separation unit 10, according to the invention, shown in FIG. 3, can be regulated by altering the proportions of the process of the enrichment and evaporation stages 27, 28 of the column 12 of the separation unit 10. In the enrichment section 27, a pressure of 0,4 bar can be used, but the pressure of the evaporation section 28 is selected using the throttle 21, in such a way that the condensation point of the oxygen fraction is lower than the condensation point of the air entering the process, which is 81,5 K at normal pressure. If pure oxygen is desired with a condensation point of 79 K, its pressure will then be 0,25 bar. Even at such a low pressure, the volume flow of the oxygen fraction will be lower than that of the air entering the process at normal pressure.

[0037] The pressure losses of the oxygen concentrate, arising in the heat exchanger, can be reduced, for example, by using the arrangement of FIG. 3. The concentrate coming from the evaporation section 28 of the column 12 is boosted to a higher pressure by a booster 30, while being simultaneously cooled in the heat exchangers 26 and 27 using the cold, oxygen-poor process air exhausting from the separation unit 11 of the column 10. The process air in question is then expanded in a cooling turbine 32, the low-pressure exhaust flow of which is led to the heat exchanger 14, together with the low-pressure nitrogen fraction exhausting from the enrichment component 27.

[0038] In the example of FIG. 4a, the double column is constructed to form a tank 10, in which there are a number of tubes 33, on the outer surface of which there are horizontal plates 34. The tubes are divided into two groups, X and Y, which are positioned, according to FIG. 4b, in such a way that each X tube is surrounded by four Y tubes, and each Y tube is surrounded by four X tubes.

[0039] The construction of the fins 33 of the tubes shown in FIG. 4a can alternatively be created from hollow disc-like components, the internal space of which is connected to the internal space of the tube. This increases the heat-transfer surface of the halves of the double column.

[0040] The plates of the X tubes lie between the plates of the Y tubes, in such a way that each plate of an X tube is in the centre of a four-sided prism formed by eight plates of Y tubes, and correspondingly each plate of a Y tube is in the centre of a four-sided prism formed by eight plates of X tubes. A regular three-dimensional labyrinth is created between the tubes and the plates attached to them, which increases the turbulence of the gas and liquid phases flowing in them and the mass and thermal transfer between the phases. Due to the regularity of the construction and the wide flow channels, it is possible to maintain higher flow velocities in it than in columns according to the state of the art. Thus, these constructions form regular, wide flow channels to improve the mass and thermal transfer between the phases and to reduce the pressure losses of the gas flows.

[0041] In this example, the space between the tubes, the A half of the double column, is the liquidization unit, the liquid oxygen fraction collecting on the bottom of which is led through a throttle valve 37 to the B half of the double column, comprising the interior space of the tubes 33, which acts as the evaporation unit. Structures 36 shaped like coil springs are located inside the tubes of the B half and guide the liquid phase flowing in the tubes into spiral paths, thus also increasing the turbulence of the gas phase flowing in the tubes and making the mass and thermal transfer between the phases more efficient.

[0042] This double column operates in the same way as the construction depicted in the first embodiment, i.e. pre-cooled process air is led at normal pressure to the bottom of the A half, from where it flows upwards while the partly liquidized oxygen-poor fraction of the process air exhausts from the connection 23 to the heat exchanger. The oxygen fraction liquidized in the A half is evaporated in the B half at a pressure of about 0,4 bar and the gaseous oxygen fraction thus obtained exits from the connection 24 at the bottom of the unit to the heat exchanger.

[0043]FIG. 5 shows an embodiment, in which most of the heat flux released during liquidization is transferred to evaporation using a heat-transfer liquid, for example, the nitrogen or oxygen fractions, or their components, which are circulated inside finned tubes 35 with the aid of a pump 20. Due to the low viscosity of the components of liquid air, this method permits the creation of an efficient thermal connection between the liquidization and evaporation units. Thus, large flow cross-sectional areas can be used in the liquidization and evaporation units and the shape and possible fillings of their flow channels can be optimized. 

1. A process for producing oxygen or air enriched with oxygen, in which process: the air entering the process is cooled in a heat exchanger to near to the condensation point of air, the air is led to a separation unit formed of two or more chambers connected thermally to each other and comprising A and B halves, in which it is divided, using liquidization and evaporation processes, into two or more gaseous fractions, and the gas fractions leaving the separation unit are led back to the heat exchanger, characterized in that the process air is maintained at normal temperature, except for small (<0.3 bar) pressure differences to maintain the flow of the air and to optimize the process, and when the process air flows through the A half of the separation unit, the liquid oxygen concentrate separated from it is evaporated in the B half of the separation unit at such a pressure, lower than normal pressure, that the desired composition is obtained for the oxygen concentrate and the heat of liquidization released in the A half of the unit is transferred to the B half of the unit and is bound in it in the evaporation of the oxygen concentrate, and the low-oxygen-content process air leaving the A half of the separation unit is led out of the process through the heat exchanger, and the oxygen concentrate evaporated in the B half of the separation unit exits at the lower than normal pressure prevailing in the half in question, either as such, or through a boosting and cooling process to the heat exchanger, and from there out of the process, and the liquidization and evaporation processes are carried out in the said separation unit in nearly reversible conditions, in such a way that the gas phase in each process and in each part of the separation unit is nearly in equilibrium with the liquid phase in the same part.
 2. A process according to claim 1, characterized in that the oxygen fraction produced by the process, either as such or boosted to a higher pressure, undergoes a new separation process described in claim 1, in order to increase its oxygen content.
 3. A process according to claim 1, characterized in that the B half of the separation unit is divided into two parts, the first of which is an enrichment section operating on the countercurrent principle, in which the liquid oxygen concentrate leaving the A half of the separation unit flows downwards, while part of it evaporates in nearly reversible conditions, in such a way that the gas phase at each height is nearly in thermal equilibrium with the liquid phase at the same level, and in which such a pressure lower than normal pressure prevails in the enrichment section, that the desired composition is obtained in the part of the oxygen concentrate leaving the bottom of the said enrichment component, and, in the part of the A half in a heat-exchange connection with said component, the heat of liquidization released in part of the enrichment section transfers to the said enrichment section and in it is bound in the evaporation of the desired part of the oxygen concentrate, and in which part of the oxygen concentrate evaporated in the enrichment section exits from the top of the section in question, at the pressure lower than normal pressure prevailing in it, through a heat exchanger into a booster, in which it is boosted to the desired pressure, and the part of the oxygen concentrate that has remained unevaporated in the enrichment section is led from the bottom of it to the top of the second part of the B half, from where it flows downwards and evaporates at such a lower than normal pressure, that heat of liquidization released in the part of the A half in heat-exchange connection with the said other half is transferred to the said second half and is bound in it in the evaporation of the said oxygen concentrate, and the said evaporated oxygen exits, either as such or through a boosting and cooling process, to the heat exchanger and from there out of the process.
 4. A process according to claim 1, characterized in that the conditions of the process are arranged to be such that a substantial part of its oxygen remains in the process air, thus reducing the energy consumption of the process.
 5. A process according to claim 1, characterized in that the oxygen fraction leaving the separation unit is boosted to a higher pressure, cooled in a separate cooling cycle, and the cooled flow is led out of the process through a heat exchanger.
 6. A process according to claim 5, characterized in that the said separate cooling cycle includes structures, in which the nitrogen fraction exiting at normal pressure from the liquidization section of the process is arranged to cool the oxygen fraction and the said nitrogen fraction is then expanded in a cooling turbine, after which the expanded flow is connected to the low-pressure flow exiting from the enrichment component of the unit to the heat exchanger.
 7. A process according to claim 1, characterized in that the thermal-transfer connection between the A and B halves of the separation unit is implemented in such a way that the halves together form a tank, in which there is a number of tubes, the space between which tubes acts as one half of the unit and the space inside which tubes acts as the other half of the unit.
 8. A process according to claim 1, characterized in that the thermal-transfer connection between the halves A and B of the separation unit is implemented in such a way that the halves form a module comprising two tubes inside each other, of which there can be one or several, and in which module the space between the inner and outer tubes can act as the A half and the interior space of the inner tube as the B half, or vice versa.
 9. A process according to claim 1, characterized in that at least part of the heat flux released in the A half of the separation unit is transferred to the B half of the unit by circulating a suitable thermal-transfer fluid in the tubes located in them.
 10. A process according to claim 9, characterized in that the thermal-transfer fluid referred to in claim 9 is a liquid fraction of oxygen or nitrogen, or a component of them.
 11. A process according to claim 1, characterized in that constructions are used in the separation unit, which form regular, wide flow channels between the phases, in order to make the mass and thermal transfer more efficient and to reduce the pressure losses of the gas flows. 